An alarming number of Windows users are needlessly opening themselves up to attacks by rootkit and worm malware by turning off the User Access Control (UAC) that comes with Windows 7 and Vista, Microsoft has reported.

How UAC was disabled on those machines is an interesting point. There are only three routes starting by using a software exploit to hit a service which has administrator privileges by default, a tactic that appears to have worked in a growing number of cases.

Any malware that gets this far can turn off UAC to stop its requests for admin privileges after every reboot coming to the attention of the user.

A second method is by tricking the user into clicking "yes" when a UAC prompt appears. A third route is to infect a PC on which UAC was already disabled, something that Microsoft thinks is now happening because a minority of users do not want UAC prompts to interrupt their use of the PC.

It is also possible for malware to infect a PC without requiring admin privileges or interacting with UAC at all, but emboldened malware writers had recently started attacking it anyway because it offers advantages in avoiding detection.

The technique had become so prevalent that Microsoft's security software, including its Security Essentials antivirus program, now monitored UAC to detect tampering acts because such an act could indicate malware.

Faulhaber doesn't say it, but as the OS vendor this will be easier for Microsoft to monitor UAC reliably than it would be for a rival antivirus vendor.

The message remains clear -- even in its imperfect state, users should keep UAC turned on and check for programs that might be interfering with it.